1. Field of the Invention
The present invention relates to a light-emitting device and an illumination apparatus using a semiconductor light-emitting element such as a light-emitting diode (LED element) as a light source.
2. Description of the Related Art
A conventional illumination apparatus which includes a plurality of LED elements connected in series-parallel as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2008-053695 is available. Also, an illumination apparatus is available which, as shown in FIG. 17, comprises a power circuit 101 and an LED module 102 constituting a light-emitting device in a housing 100 of the illumination apparatus proper. In this conventional apparatus, the power circuit 101 is connected to a commercial power supply 104 through a power switch 103, and by the switching operation of a switching element, not shown, with the power switch 103 turned on, controls the DC output to the LED module 102. The LED module 102 is configured of a plurality of series-connected LED elements 105 mounted on the surface of a printed board 106 and adapted to be turned on with the LED elements 105 as a light source by the DC output of the power circuit 101.
The illumination apparatus of this type is used with the housing 100 grounded for protection against an electric shock, etc. In this case, the printed board 106 with the LED elements 105 mounted thereon is fixed closely on the inner wall of the housing 100 taking the heat radiation into consideration, and therefore, stray capacitance 107 exists between the printed board 106 and the housing 100. If a thin printed board is used to improve the efficiency of radiating the heat generated by the LED elements, the stray capacitance increases. The stray capacitance further increases especially for an LED module in which an insulating layer is formed on the surface of a metal board and the LED elements 105 are formed thereon.
In the case where a common-mode noise (with the ground) is generated by an external noise source 108 such as a high-frequency variation or an impulse noise which causes the instability of the grounding potential, therefore, a noise current may flow into the LED elements 105 through the stray capacitance 107. Especially, in the case where the power switch 103 of single-side type as shown is used, the noise current may continue to flow into the LED elements 105 through the path indicated by a dashed line a and may erroneously turn on the LED elements 105 in spite of the off state of the power switch 103. Also, in the case where the LED elements 105 of the illumination apparatus having the dimmer function are turned on with a deep (dark) dim light, the problem is sometimes posed that the current flowing into the LED elements 105 due to the common-mode noise may cause a flicker, thereby having an extremely adverse effect on the commercial value.
A solution to this problem has been conceived in which a bypass capacitor is connected in parallel to each of the series-connected LED elements 105 to provide a bypass for the current flowing through the stray capacitance 107 due to the common-mode noise.
With the increase in the number of the series-connected LED elements 105 and hence the number of the capacitors connected in parallel to the LED elements 105, however, the fact that the plurality of the capacitors are connected substantially in series decreases the combined capacitance of the capacitors as a whole and increases the AC impedance against the ground, resulting in a reduced bypass effect. Thus, the LED elements 105 connected to a higher potential side would be erroneously turned on by the current inflow due to the common-mode noise. The use of a large-capacitance capacitor may be considered to solve this problem. A capacitor of large capacitance, however, increases both the size and cost of the apparatus, and would lead to bulkiness and a high price of the whole illumination apparatus.
As a method of reducing the effect of the common-mode noise on the LED elements, an insulation-type switching transformer may also be used for the power circuit 101. In the insulation-type switching transformer, however, a capacitor is inserted between the primary and secondary windings to suppress noises, and therefore, the effect of the common-mode noise cannot be completely eliminated. Further, the use of the insulation-type switching transformer poses the problem of a bulky and expensive power circuit.